android jetpack livedata postValue(), setValue()
original source : https://blog.mindorks.com/livedata-setvalue-vs-postvalue-in-android
we can change the UI of an application when there is a change in some data.
livedata를 통해 쉽게 데이터 변경을 ui에 적용할수 있다.
livedata 설명
LiveData is an observable data holder class. Unlike a regular observable, LiveData is lifecycle-aware, meaning it respects the lifecycle of other app components, such as activities, fragments, or services.
In a simple way, LiveData is basically a data holder and it is used to observe the changes of a particular view and then update the corresponding change.
So, LiveData is used to make the task of implementing ViewModel easier. The best part about LiveData is that the data will not be updated when your View is in the background and when the view will come in the foreground, then it will get the updated data only.
Difference between setValue() and postValue()
either you can update the data on the Main thread or you can update the data using the background thread. So, the use-case of setValue and postValue depends on these two situations only.
While using the Main thread to change the data, you should use the setValue method of the MutableLiveData class and while using the background thread to change the LiveData, you should use the postValue method of the MutableLiveData class.
So, the duty of the postValue method is to post or add a task to the main thread of the application whenever there is a change in the value. And the value will be updated whenever the main thread will be executed (여기서 mainthread가 실행될때 이부분이 중요한 부분). So, basically, it is requesting the main thread to set the new updated value and then notify the observers.
While the setValue method is used to set the changed value from the main thread and if there are some live observers to it, then the updated value will also be sent to those observers as well. This setValue method must be called from the main thread.
So, here are some of the points that you must think before using setValue and postValue:
If you are working on the main thread, then both setValue and postValue will work in the same manner i.e. they will update the value and notify the observers.
If working in some background thread, then you can't use setValue. You have to use postValue here with some observer. But the interesting thing about postValue is that the value will be change immediately but the notification that is to be sent to observers will be scheduled to execute on the main thread via the event loop with the handler.
Let’s take an example,
// setValue liveData.setValue("someNewData") liveData.setValue("againNewData") // postValue liveData.postValue("someNewData") liveData.postValue("againNewData")
In the above example, the setValue is called from the main thread and the postValue is called from some background thread.
Since the setValue is called twice, so the value will be updated twice and the observers will receive the notification regarding the updated data twice.
But for postValue, the value will be updated twice and the number of times the observers will receive the notification depends on the execution of the main thread. For example, if the postValue is called 4 times before the execution of the main thread, then the observer will receive the notification only once and that too with the latest updated data because the notification to be sent is scheduled to be executed on the main thread. So, if you are calling the postValue method a number of times before the execution of the main thread, then the value that is passed lastly i.e. the latest value will be dispatched to the main thread and rest of the values will be discarded.
Another thing to care about postValue is that, if the field on which the postValue is called is not having any observers and after that, you call the getValue, then you will not receive the value that you set in the postValue because you don't have an observer here.
